JPH0924378A - Precipitating and removing method of phosphorus compound in waste water - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently precipitate phosphorus acid or its salt incorporated in a waste water as a calcium salt good in filtering property and to remove the salt by filtering. SOLUTION: The phosphorus acid or its salt in the waste water is precipitated as a coarse crystal similar to hydroxyapatite or as an amorphous solid. The phosphate incorporated in the liq. is oxidized to form orthophosphate and the orthophosphate is crystallized as the hydroxyapatite. A respective individual liq. obtained by this method or a solution containing the crystals similar to those of the hydroxyaptite or that containing the amorphous solid and the hydroxyapatite crystals are filtered.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for precipitating phosphorous acid or its salt in waste water as a calcium salt and removing it by filtration, and a method of adding an oxidizing agent to the precipitation liquid or the filtrate to dissolve it. The present invention also relates to a method of almost completely precipitating phosphite, filtering and removing it.

[0002]

2. Description of the Related Art Phosphorus trichloride, which is used for various purposes in the chemical industry, exists as phosphorous acid or its salt in waste water. On the other hand, the concentration of phosphorus in wastewater is strictly regulated from the viewpoint of environmental protection, and is generally 1 mg / l or less to 20 mg / l.
All of the following are instructed to make the concentration extremely low. Therefore, in the factory, it is necessary to remove phosphorus from the wastewater and reduce the concentration to below the standard value of wastewater before discharging. The conventional technology for removing phosphorous acid or its salts in wastewater is to oxidize it with sodium hypochlorite to orthophosphoric acid or its salts, which is then aggregated with lime, calcium chloride, aluminum sulfate or ferric chloride. A method is known in which coagulation and precipitation are performed with an agent, the precipitate is filtered, and the resulting filtrate is discharged.

[0003]

According to the prior art,
A step of oxidizing all amount of phosphorous acid or a salt thereof with sodium hypochlorite, and a step of reducing the excess amount with a reducing agent such as sodium acid sulfite when the amount used is excessive,
It is necessary to add one or two steps. Further, since the precipitate is non-crystalline, it is difficult to filter it, and the operation requires a long time, and the water content of this product is as high as about 70% or more. Therefore, the amount of the filtrate is large and the handling is difficult. Difficult,
Further, the processing cost will be considerable.

From the viewpoint of equipment, one or two reaction tanks and one or two storage tanks are required for the tanks alone. In particular, the storage tank of sodium hypochlorite liquid is a strong oxidizer and is unstable, so it is necessary to take serious safety measures for the equipment. From a cost perspective, in addition to the above equipment costs, labor costs,
The raw material cost and the utility fee are additionally required, and the total of these costs imposes a considerable financial burden.

[0005]

As a method for removing orthophosphoric acid or a salt thereof, which is handled in a large amount in the industry related to phosphorus, from wastewater, it is reacted with a calcium compound to form a sparingly soluble material called hydroxyapatite. A method of depositing and removing calcium salt crystals is known. The crystals are coarse crystals and the filtration is easy. According to the present invention, when the hydroxyapatite precipitation method is applied to phosphorous acid or a salt thereof, a hydroxyapatite-like substance is produced,
It was found that there were hydroxyapatite-like crystals which were easy to filter and amorphous solids which were not easily filtered, and therefore, it was attempted to remove phosphorous acid or a salt thereof from waste water by utilizing this precipitation method. The following three methods can be given as specific procedures for obtaining hydroxyapatite-like crystals.

As one method, about 2 moles of calcium compound is added to and dissolved in the waste water, and this solution is added in an acidic state, for example, p.
A seed crystal is added with H of about 4, and an alkali metal hydroxide such as sodium hydroxide solution is gradually added with stirring to raise the pH slowly for precipitation. In the second method, the pH of waste water is adjusted to, for example, about 6.0, seed crystals are added, and then a portion of about 2 mol of calcium chloride solution is gradually added with stirring to lower the pH to about 5.0. Then, a sodium hydroxide solution is gradually added to restore the original pH to about 6.0. This pH of 6.0 to 5.
Alternating additions between 0 are repeated at least 6-7 times to precipitate. As the third method, the pH of the waste water is adjusted to 5.5, for example, and seed crystals are added, and then about 2 moles of calcium chloride solution and sodium hydroxide solution are gradually added simultaneously while maintaining this pH while stirring. , Precipitate.

On the other hand, the hydroxyapatite-like crystals produced from the solution of phosphorous acid crystals are filtered, and the concentration of phosphorus in the mother liquor obtained is a minimum of 20-40 mg / l near pH 7-8. It has been found that the concentration of phosphorus increases both at lower and higher pH.
In the present invention, based on this finding, the concentration of phosphorus in the filtrate is reduced as much as possible. That is, all of the precipitation liquids obtained by the above-mentioned three methods bring the pH of the liquid to the 8th position at the final stage and sufficiently precipitate hydroxyapatite-like crystals. The phosphorus concentration in the filtrate is about 20 to 100 mg / l.

If the phosphorus concentration is lower than the standard value of waste water, the filtrate is discharged. If it is higher, an oxidizing method using an oxidizing agent such as sodium hypochlorite is applied to the filtrate to precipitate phosphite as hydroxyapatite. The phosphorus concentration in the filtrate is 0 to several mg / l. In addition, when it is expected in advance that it is higher than the standard value of waste water, this oxidation method is applied to the precipitation liquid before filtration.

The water content of the filtration product obtained by the coagulation-sedimentation method described above is about 70%. On the other hand, the water content of the filter cake obtained by the present invention is around 40%, and when it is lower, it may decrease to 30%. According to the invention, the weight of the wet cake is reduced by almost half.

The individual conditions of the present invention will be described in more detail. The phosphorus concentration in various waste water containing phosphorus trichloride is several tens.
It is distributed in the range of 0 mg / l to several%. In this,
Those in the range of a few 1000 mg / l to about 2% are desirable for the application of the present invention. Next, the weight percentage of phosphorous acid-derived phosphorus in the total phosphorus (the total amount of phosphorus derived from orthophosphoric acid and the amount of phosphorus derived from phosphorous acid) in various wastewater is often about 40% to 100%, The present invention can be applied to any weight ratio.

The calcium compound used in the present invention is most preferably a water-soluble calcium compound such as calcium chloride. A sparingly soluble calcium compound such as calcium oxide or calcium hydroxide can be used as long as it dissolves in the presence of phosphorous acid, orthophosphoric acid or hydrochloric acid. Regarding the amount of calcium, a calcium amount of about 1.7 mol or more, particularly preferably about 2 mol, is required with respect to phosphorous acid. Even if the amount of calcium was increased to 3 mol and 4 mol, the phosphorus concentration in the filtrate remained almost unchanged. Regarding the amount of 1.7 mol or less, in the case of hydroxyapatite, the molecular formula Ca
_{From the} viewpoint of ₁₀ (PO ₄ ) ₆ (OH) ₂ , 1.7 moles of calcium are theoretically necessary, but from this viewpoint, the lower limit of calcium in the case of phosphorous acid is estimated as described above. Examples of the alkali metal hydroxide used in the present invention include sodium hydroxide and potassium hydroxide.

In order to obtain a large crystal similar to hydroxyapatite, it is desirable to carry out the precipitation in the strongest acidic condition in the above three methods, and the precipitation time is preferably as long as possible. Considering the time, it seems that about 2 hours is appropriate. It is also effective to add the crystal as a seed, and this serves as a nucleus to generate and grow a crystal. The amount is about 1/10 of the amount of precipitation.
It is estimated that 0 amount to 1/10 amount is sufficient.

The liquidity at the time when hydroxyapatite-like crystals start to precipitate depends on the concentration of phosphorous acid, but is generally around pH 3 to 6, and the higher the concentration of phosphorus, the higher the p
H becomes low. When the crystals start to precipitate, the pH drops by 0.5 to 1.0. This decrease is the same phenomenon as in the case of orthophosphoric acid, and therefore it is presumed that the crystal also has a hydroxyl group like hydroxyapatite.

When the above method is applied to waste water in the present invention, an amorphous solid is produced when the impurity content is relatively high, the precipitation time during precipitation is short, or the pH during precipitation is high in alkalinity. Is generated. In this case, the phosphorus removal rate is almost the same as in the case of hydroxyapatite-like crystals, but it is not easy to filter the precipitation liquid containing the crystals, and the water content of the resulting filter is as high as about 70%.

[0015]

Example 1 A 5N potassium hydroxide solution is added to an acidic waste water 11 having a phosphorus concentration of 1000 mg / l (phosphorus acid-derived phosphorus of 1,000 mg / l) to adjust the pH to 3. To this, 20 g of 35% calcium chloride solution is added, and 5N potassium hydroxide solution is gradually added dropwise with stirring. When the pH of the solution reached 4.0, 0.5 g of seed crystals were added and 60 ml of 5N potassium hydroxide solution was added.
The pH is 5.0 during the 10 minutes of dropwise addition at the speed of / hour.
Hydroxyapatite-like crystals start to precipitate from the vicinity. After completion of dropping, the solution is stirred for 10 minutes. This operation of drop deposition and stirring was repeated about 6 times, and finally the pH of the liquid
To 8.0. Since this pH tends to decrease for a while, the pH is adjusted to 8.0 each time, and the precipitation operation is terminated when this value is held for 30 minutes. 1040 ml of this deposited liquid is filtered under aspirator reduced pressure with Nuttsue having a diameter of 9.5 cm. Filtration time 1.5 minutes, filtrate phosphorus concentration 27 mg / l, wet filter material 9.5 g, 24 hours 15
5.5 g of the dried product after drying at 0 ° C.

[0016]

Example 2 Phosphorus concentration of 5,000 mg / l (phosphorus acid-derived phosphorus 3,750 mg / l + orthophosphoric acid-derived phosphorus 1,250 mg / l
The acidic wastewater 11 of 1) is neutralized with a 10N sodium hydroxide solution to pH 6.0. 2.5 g of seed crystals were added to this, and 100 g of 102 g of 35% calcium chloride solution was added with stirring.
When dropped at a rate of / hour, crystals gradually precipitate,
When H reaches 5.0, the dropping is stopped. Next, 10N sodium hydroxide solution is added dropwise at a rate of 60 ml / hour to return the pH of the solution to the original pH of 6.0. After that, this alternate dropping operation is repeated. After that, pH adjustment and filtration are performed as in Example 1. Filtration time of 2 minutes, phosphorus concentration of the filtrate 93 mg / l, wet filter cake 42.0 g, dry filter cake 26.5 g. 1200 ml of this filtrate
With stirring, 5% chlorine effective sodium hypochlorite liquid 30m
1 and then stirred for 1 hour. Although the pH of the liquid gradually decreases, the pH is adjusted to 8.0 each time, and after confirming that the pH of the liquid does not decrease for 30 minutes, this is filtered. The phosphorus concentration of the filtrate is 0 mg / l.

[0017]

Example 3 Phosphorus concentration 5,000 mg / l (phosphorous acid-derived phosphorus 2,500 mg / l + orthophosphoric acid-derived phosphorus 2,500 mg / l
The acidic waste water 11 of 1) is neutralized with a 10N sodium hydroxide solution to pH 5.5. To this, 2.5 g of seed crystals were added, and 102 g of 35% calcium chloride solution and 10N sodium hydroxide solution were simultaneously stirred under stirring for 2 hours to reach pH 5.5.
While maintaining the temperature, the solution is dropped to precipitate crystals. PH of deposit
Is increased to 7.0, and 30 ml of 5% chlorine-effective sodium hypochlorite liquid is added to this liquid with stirring, and the mixture is stirred for 1 hour. After that, pH adjustment and filtration are performed as in Example 1. Filtration time 3
Minutes, the phosphorus concentration of the filtrate was 3 mg / l, the wet residue was 40.9 g, and the dry residue was 26.5 g.

[0018]

According to the present invention, phosphorous acid or a salt thereof in waste water can be directly deposited as a hydroxyapatite-like substance. When this product is a hydroxyapatite-like crystal, the filtration operation becomes easy, the time is also completed in a short time, and the amount of the wet filter material can be reduced to about half. Further, even if it is used for removing a small amount of residual phosphorous acid without using sodium hypochlorite which has been conventionally required, it is not necessary to use the conventional sodium hydroxide.
It can be reduced from one-hundredth to hundredths.
From the above, it is possible to significantly reduce the cost required for the actual wastewater treatment, including equipment, raw materials, utility, and in some cases, labor hours.

[0019]

[Brief description of drawings]

FIG. 1 shows an example of an apparatus used in the method of the present invention.

[Explanation of symbols]

 1 Calcium chloride tank 2 Sodium hydroxide tank 3 Hydrochloric acid tank 4 Precipitation tank 5 ・ ・ ・ Filter 6 ・ ・ ・ Neutralization tank 7 ‥

Claims (2)

[Claims] 1. A calcium compound and a hydroxide of an alkali metal are added to waste water containing phosphorous acid or a salt thereof to precipitate a hydroxyapatite-like substance, and the pH of the precipitation liquid is adjusted to around neutral or weakly alkaline and filtered. A method for depositing and removing phosphorous acid or a salt thereof, which comprises: 2. A calcium compound and an alkali metal hydroxide are added to wastewater containing phosphorous acid or a salt thereof to precipitate a hydroxyapatite-like substance, and the phosphite in the precipitation liquid is oxidized with an oxidizing agent. A method for depositing and removing phosphorous acid or a salt thereof, which comprises reacting the obtained orthophosphate with a calcium compound to deposit as hydroxyapatite, and filtering the pH of the deposit to be around neutral or weakly alkaline and filtering.